The present invention relates to a quartz element, quartz resonator, quartz resonator unit, and oscillator using these component elements and particularly to an AT-cut quartz element having a rectangular shape for use in an overtone oscillating operation. Furthermore, the present invention relates to a production method suitable for producing such a quartz element, quartz resonator, and quartz resonator unit.
Quartz resonator units perform their oscillating operation based on the piezoelectric effect of single crystal quartz. They can provide very stable oscillation at a constant frequency and are used in a wide variety of applications. In particular, quartz resonator units and quartz oscillators are used as reference clock generators in various electronic systems, such as communications systems, electronic data processing systems, etc. Recent trends in these electronic systems are toward smaller sizes, lighter weights, higher operating frequencies, and higher operating speeds. These trends strongly demand quartz resonator units having a smaller size and lighter weight and also having the capability of oscillating stably at a higher frequency.
AT-cut quartz elements cut out from a single-crystal quartz exhibit excellent frequency-temperature characteristics over a wide temperature range. Furthermore, AT-cut quartz elements also show a very small variation in frequency in long-term operation. The AT-cut quartz element is shaped into a rectangular form having a length l in the X-axis, thickness t in the Yxe2x80x2-direction, and width w in the Zxe2x80x2-direction, wherein the length l is greater than the other dimensions, and placed in a small-sized quartz holder having a cylinder shape, which is sealed. For this reason, together with the excellent characteristics described above, the AT-cut quartz element is advantageously used to construct a small-sized high-performance quartz resonator unit. The X-axis, Y-axis, and Z-axis refer to the electrical axis, mechanical axis, and optical axis, respectively, of a single crystal quartz, and the Yxe2x80x2-axis and Zxe2x80x2-axis refer to the Y-axis and Z-axis rotated about the X-axis by about 35xc2x0.
To provide an oscillator using a quartz resonator unit in the form of a surface mounting device (SMD), which can be mounted on a circuit board in the same manner as ICs, it is required that the quartz resonator unit is accommodated in a small-sized holder having a diameter of 2 mm or less and a length of about 6 mm so that the oscillator may be used in conjunction with advanced integrated circuits having a small size. As reported in the 21st EM symposium (Papers of Technical Meeting on Electronic Circuits, IEEJ, pp. 37-42, May 22, 1992), quartz elements having such a small size that can be accommodated in a quartz holder having a cylindrical form so that they can oscillate at a fundamental frequency have been achieved already. However, quartz resonator units that oscillate at a fundamental frequency can cover only a low frequency range, such as 17 MHz to 40 MHz, and they cannot be used in high frequency bands greater than 40 MHz required in high-speed electronic systems such as those described above.
The oscillation frequency of an AT-cut quartz element varies inversely with its thickness t. Therefore, if the fundamental frequency exceeds 40 MHz, then the thickness of the quartz element becomes less than 42 xcexcm, and thus, production becomes very difficult. Therefore, to realize quartz resonator units that can oscillate at high frequencies, it is necessary to develop a quartz element that can be used in the overtone oscillation mode and a quartz resonator unit using such a quartz element. To accommodate a quartz element in such a small-sized holder as described above, its length l should be less than 5 mm, and its width w should be less than 1.5 mm. However, if an AT-cut quartz element having such a small size is used for overtone oscillation, spurious vibration often occurs near the thickness shear mode of the main vibration. Furthermore, coupling occurs between the spurious vibration and the main vibration, and an even small temperature change, such as 5xc2x0 C.-10xc2x0 C., causes a jump in oscillation frequency. Thus, in the case of a quartz element having such a small size, an optimum shape of the quartz element, especially regarding the width-to-thickness ratio (the ratio of width w to thickness t), is not known yet that gives a cubic-curve frequency-temperature characteristic, which is essential in AT-cut quartz resonator units, in the required temperature range (about xe2x88x9220xc2x0 C. to about +80xc2x0 C.).
In quartz resonators using a small quartz element, energy trapping of the thickness shear mode of the main vibration is often insufficient, which causes degradation in resonance resistance Rr. In small-sized quartz elements and quartz resonators, especially such ones used in the overtone oscillation mode, the dependencies of their dimensions, the surface roughness, the width of electrodes, and the weight of electrodes on the resonance resistance Rr are not known.
In surface processing, if the surface roughness is improved, it is possible to decrease the resonance resistance Rr in processing. However, the resonance resistance Rr varies from product to product. Therefore, in usage of such a quartz element having a small size, just a simple improvement in the surface roughness does not lead to practical production of high-performance quartz elements at low cost, since it is impossible to obtain a sufficiently high yield in production.
Thus, it is an object of the present invention to provide a quartz element, quartz resonator, and quartz resonator unit having a sufficiently small size and light weight so that they can be used in the form of SMDs, like ICs, and thus provide a quartz oscillator using such elements. Thus, more specifically, it is an object of the present invention to provide a quartz element having a length l less than about 5 mm and a width w less than about 1.5 mm that is shaped such that it can oscillate in the overtone mode with good temperature characteristics. It is a further object of the present invention to provide a quartz resonator unit using such a small-sized quartz element, or quartz resonator, that has a low resonance resistance Rr and thus can be used in practical applications. It is further object of the present invention to provide a method of producing, with a high production yield, a quartz element, quartz resonator, and quartz resonator unit having excellent temperature characteristics and a low resonance resistance Rr.
To achieve such a quartz resonator unit having a small size and also having the capability of oscillating at a high frequency, the inventor of the present invention has performed experiments and evaluations repeatedly, and finally succeeded in achieving a small-sized quartz element for use in overtone oscillations, which exhibits no coupling with spurious vibrations over the entire temperature range in which a quartz resonator unit using the quartz element is expected to operate. Such a quartz element is an AT-cut quartz element shaped in a rectangular form for use in a third overtone quartz resonator unit, characterized in that it has a length l along the X-axis, a thickness t along the Yxe2x80x2-axis, and a width w along the Zxe2x80x2-axis, wherein the width-to-thickness ratio w/t is in a range selected from the group consisting of 8.48xc2x10.05, 12.18xc2x10.05, 13.22xc2x10.07, 14.78xc2x10.07, and 15.57 0.07.
The inventor of the present invention, after further experiments and evaluation, has also succeeded in achieving a quartz element characterized in that it has a length l, along the X-axis, in the range of 4000 xcexcm to 4700 xcexcm and a width w in the range of 800 xcexcm to 1500 xcexcm so that it can form a quartz resonator unit having an excellent resonance resistance.
Furthermore, the inventor of the present invention has succeeded in achieving a quartz element characterized in that its surface is etched such that the maximum height Rmax of its surface roughness is in the range of 0.2 xcexcm to 0.7 xcexcm or, more preferably, in the range of 0.3 xcexcm to 0.6 xcexcm so that it has an excellent resonance resistance. In conventional quartz elements, the surface is simply processed such that it becomes as flat as possible thereby reducing the resonance resistance. In contrast, in the present invention, the surface roughness is controlled in the range described above so as to achieve not only a low resonance resistance, but also an extremely high production yield.
In the present invention, such quartz elements are produced by cutting a quartz crystal into the form of an AT-cut wafer and further lapping and etching the surface of the wafer. In the production, it is preferable that the reduction in the thickness per surface resulting from the etching process, that is, the half of the total reduction in the thickness (hereafter, referred to as etched thickness) be in the range of 0.5 xcexcm-2.5 xcexcm, and furthermore, it is preferable that the maximum height Rmax of the surface roughness be in the range of 0.3 xcexcm-0.7 xcexcm at the stage just before the etching process. In the finish lapping process performed just before the etching process, it is effective that the lapping is performed using an alumina-based abrasive having an average grain size of 2.5 xcexcm-3 xcexcm. The etching process can be performed using 10-30 wt % hydrofluoric acid as an etchant.
Regarding the electrodes formed on the opposite surfaces separated by the thickness of the quartz element, the quartz element having the width-to-thickness ratio w/t in the range of 8.48xc2x10.05 can have excellent resonance resistance and temperature characteristics if the electrodes are formed such that the width W of the electrode, measured in the direction along the Zxe2x80x2-axis, is smaller than the width w of the rectangular AT-cut quartz element, wherein the spaces between the edges of the width of the electrode and the edges of the width of the AT-cut quartz element are in the range from 75 xcexcm to 230 xcexcm or, more preferably, in the range from 75 xcexcm to 200 xcexcm. Similarly, rectangular AT-cut quartz elements having the width-to-thickness ratio w/t in the range of 12.18+/xe2x88x920.05, 13.22+/xe2x88x920.07, 14.78+/xe2x88x920.07, or 15.57 +/xe2x88x920.07, the rectangular AT-cut quartz elements exhibit excellent characteristics when the above-described spaces between the electrode edges and the quartz element edges are in the range of 75 xcexcm to 340 xcexcm or, more preferably, 75 xcexcm to 200 xcexcm. Regarding the thickness of electrode films deposited by means of, for example, evaporation, an excellent resonance resistance characteristic can be obtained if the thickness is controlled in such a range that the change in the oscillation frequency of the rectangular AT-cut quartz element having deposited electrode films, relative to the frequency of the rectangular AT-cut quartz element having no electrodes, is in the range of 7000 to 30000 PPM.
If such a rectangular AT-cut quartz element is used to form a quartz resonator unit, then it is possible to obtain a quartz resonator unit having a small size and light weight and having the capability of oscillating at a high frequency. As for a supporting mechanism for supporting a quartz element, the quartz element can be supported by lead wires connected to one end of each electrode of the quartz element at its one end along the X-axis, wherein the connection may be formed by means of soldering or a conductive adhesive. Furthermore, the quartz element fabricated in the above-described manner can be small enough to be accommodated in a holder having a diameter of 2.0 mmxc2x10.2 mm and a length of 6.0 mm+/xe2x88x920.5 mm, thereby achieving stable oscillation at a high frequency. The quartz holder may be molded with a molding material. Furthermore, the quartz holder may also be molded together with an integrated circuit including an oscillation circuit, thereby achieving a quartz oscillator suitable to be mounted on the surface of a circuit board.
Each element and its structure will be described below in more detail referring to preferred embodiments of the invention. However, it should be understood that the invention is not limited to these specific embodiments of quartz elements, quartz resonators, or quartz resonator units, except as defined in the appended claims.